An integrated circuit has been developed, which uses a semiconductor substrate so-called a silicon-on-insulator (hereinafter also referred to as an SOI) that has a thin single crystal semiconductor layer over an insulating surface, as an alternative to a silicon wafer that is manufactured by thinly slicing an ingot of a single crystal semiconductor. The integrated circuit using the SOI substrate attracts much attention because the use of the SOI substrate enables the reduction of parasitic capacitance between a drain of a transistor and the substrate, which contributes to the improvement in performance of a semiconductor integrated circuit.
There are various methods for manufacturing an SOI substrate, and Smart Cut (registered trademark) is known as a method which allows easy production (high throughput) of SOI substrate having a SOI layer with high quality. The SOI substrate is formed in the following manner: hydrogen ions are implanted into a base wafer which is to be a base of a silicon layer, and the resulting wafer is bonded to another wafer (a bond wafer) at room temperature. A strong bond can be formed between the base wafer and the bond wafer even at room temperature due to van der Waals force. The base wafer bonded to the bond wafer is subjected to heat treatment at a temperature of about 500° C., and a silicon layer is separated from the base wafer utilizing a layer to which the hydrogen ions are implanted as a boundary.
As a method for forming a single crystal silicon thin film obtained by utilizing Smart Cut (registered trademark) over a crystallized glass, which is a high heat resistant glass, a method invented by the applicant of the present invention is known (Patent Document 1: Japanese Published Patent Application No. H11-163363).
Further, a thin film transistor which is included in a driving circuit and a control circuit of a display, such as a liquid crystal display and an organic EL display, is formed of an amorphous silicon film, a polycrystalline silicon film, or the like which is formed over a glass substrate. In recent years, demand for higher definition or higher speed driving of the display has been increased. Therefore, many attempts have been made to form a single crystal silicon film with higher carrier mobility over a glass substrate.
With recent increase in the size of a screen and improvement of productivity by increasing the number of panels obtained from one substrate, the size of mother glass has been increased. For example, although practice application has not been made, a mother glass whose size is 2850 mm×3050 mm (so-called tenth generation) is known as the largest mother glass at the present time.
On the other hand, the largest size of a silicon wafer is 300 mm in diameter. Therefore, in order to form an SOI layer throughout a surface of a large glass substrate a plurality of silicon wafers is required to be bonded. In this case, it is difficult to manufacture a large SOI substrate over the large glass substrate without the formation of a joint portion (gap) between adjacent SOI layers.